System access and synchronization methods for MIMO OFDM communications systems and physical layer packet and preamble design

ABSTRACT

A method and apparatus are provided for performing acquisition, synchronization and cell selection within an MIMO-OFDM communication system. A coarse synchronization is performed to determine a searching window. A fine synchronization is then performed by measuring correlations between subsets of signal samples, whose first signal sample lies within the searching window, and known values. The correlations are performed in the frequency domain of the received signal. In a multiple-output OFDM system, each antenna of the OFDM transmitter has a unique known value. The known value is transmitted as pairs of consecutive pilot symbols, each pair of pilot symbols being transmitted at the same subset of sub-carrier frequencies within the OFDM frame.

RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. provisionalapplication Nos. 60/329,507, 60/329,510 and 60/329,514 all filed Oct.17, 2001.

FIELD OF THE INVENTION

[0002] This invention relates to cellular wireless communicationsystems, and more particularly to system access within cellular wirelesscommunication systems employing OFDM or OFDM-like technology, and tophysical layer packet and preamble designs.

BACKGROUND OF THE INVENTION

[0003] In a wireless communication system having at least onetransmitter and at least one receiver, the receiver must acquire thetiming of a signal transmitted by the transmitter and synchronize to itbefore information can be extracted from the received signal. The timingof signals transmitted from a base station, within a wirelesscommunication system, is commonly referred to as the system timing.

[0004] In cellular wireless communication systems employing OrthogonalFrequency Division Multiplexing (OFDM), synchronization to the timing ofa signal enables the exact positioning of a Fast Fourier transform (FFT)window utilised by a receiver of the signal to extract information fromthe signal.

[0005] In any cellular wireless communication system having multiplebase stations (BTS) and multiple mobile communication devices thesynchronization process must occur frequently between the BTS and themobile communication devices for the system to be operable. The mobilecommunication devices will simply be referred to hereinafter as UE (userequipment).

[0006] Furthermore, each BTS defines a geographic transmission region,known commonly as a cell, in which UE in substantially close proximityto a particular BTS will access the wireless communication system. Theprocess whereby a particular UE selects a BTS from which to access thecellular wireless communication system is known as cell selection. Inorder to optimize the reception of the BTS signal, the UE needs toidentify the best quality signal received from different BTSs and switchits receiver to tune into the best BTS for a given time. Thus, due tothe mobility of UE, the synchronization process has to be employedfrequently in order to allow seamless handoffs from one BTS to anotherBTS as the UE changes location.

[0007] In most current cellular wireless communication systems, fastsystem access and cell selection are essential functions for propermobile UE operation. The objective of fast acquisition is to allow UE tosynchronize into the desired BTS. The cell selection and re-selection isperformed by UE to synchronize and measure the signal (including theinterference) power among the adjacent BTS and select and switch to theBTS with the best signal quality, namely the maximum C/I(carrier-to-interference) ratio.

[0008] Existing solutions to access a wireless communication systememploying OFDM (Orthogonal Frequency Division Multiplexing) weredesigned for wireless LAN (local area network) systems for fast packetaccess under a SISO (single input-single output) configuration. However,the wireless LAN does not have the capability to deal with the UEmobility, which requires seamless BTS handoff. On the other hand somecellular systems e.g. 3G UMTS are capable of performing cell selectionand BTS identification and BTS C/I ratio measurement.

[0009] Multiple Input Multiple Output-Orthogonal Frequency DivisionMultiplexing (MIMO-OFDM) is a novel highly spectral efficient technologyused to transmit high-speed data through radio channels with fast fadingboth in frequency and in time. For a high-speed downlink packet datatransmission system, the design of the physical layer packet structureis a fundamental aspect.

[0010] OFDM technology has been adopted by DAB, DVB-T and IEEE 802.11standards. DAB and DVB-T are used for audio and video territorialbroadcasting. In these systems, the signal is transmitted in acontinuous data stream. A preamble is not needed because fast packetaccess is not critical. DAB and DVB-T are also applied in singlefrequency networks. In this case, every transmitter transmits the samesignal as a simulcast. The interference from the neighbouringtransmitters can be treated as an active echo, which can be handled bythe proper design of the prefix. IEEE 802.11 is the wireless LANstandard. It is a packet based OFDM transmission system. A preambleheader is introduced in this standard.

[0011] Synchronization within MIMO-OFDM (Multiple Input MultipleOutput-OFDM) systems, in which each transmitter and each receiver havemultiple antennae, is even more difficult. Adding to the complexity ofthe task is that a fast synchronization process must be very reliable atvery low C/I ratio conditions to allow a high rate of success for theentire cell. In addition, high mobility causes a high Doppler spread andthis makes reliable synchronization even more difficult.

[0012] In MIMO-OFDM systems, synchronization can be performed in twosteps. First, frame synchronization (also called coarse synchronization)is performed in order to determine the approximate range of the locationof the starting position of the first OFDM symbol in the frame. Second,timing synchronization (also called fine synchronization) is performedto determine the precise FFT window location, so that demodulation inthe frequency domain can be performed accurately.

[0013] Conventionally, fine synchronization is implemented in timedomain. This is achieved by inserting an a priori known pilot trainingsequence in the time domain for the receiver to perform the crosscorrelation computing at select time slots.

[0014] For example, as shown in FIGS. 1A and 1B, the OFDM framestructure of the IEEE 802.11 standard utilizes several repeated shortOFDM symbols generally indicated at 5 arranged as several headers in thetime domain at the beginning of the frame for select sub-carriers,followed by training OFDM symbols 207 for fine synchronization. Theheaders 5 are used for frame (i.e. course) synchronization. The trainingOFDM symbols 207 are used to position the FFT window precisely so thatdemodulation in the frequency domain can be performed accurately. Thetraining OFDM symbols 207 are followed by a TPS OFDM symbol 205 and dataOFDM symbols 30.

[0015] The TPS (transmission parameter signalling) OFDM symbol 205,shown more clearly in the frequency domain (see FIG. 1B), is transmittedwith a frequency that corresponds to an adaptive coding and modulationperiod. The training OFDM symbols, TPS OFDM symbol and data OFDM symbolsuse all sub-carriers. In the 802.11 system, the repeated headers forcourse synchronization are only transmitted on every fourth sub-carrier.This design is only suitable for a simple SISO OFDM system with only asingle transmit antenna. For MIMO-OFDM system the preamble design ismore complicated because of the existence of multiple transmit antennas.Furthermore for mobile communications, an efficient preamble design iseven more difficult because of the multi-cell environment, therequirement for initial access when no BTS information is available, BTSswitching and even soft handoff.

[0016] Existing methods in the process of cell acquisition andsynchronization employ a 3-step-synchronization approach adopted by UMTSWCDMA system, which requires a relatively long access time. While finesynchronization may be performed in the time domain, theself-interference of MIMO channels limits the performance of thisapproach under very low C/I conditions. Increasing the length of thecorrelation can enhance the performance of fine synchronization in thetime domain but at the price of an increase in overhead and processingcomplexity. The existing designs are based on the time domain trainingsequence correlation for a single transmit antenna and a single receiveantenna system. However, a straightforward extension of such a timedomain synchronization approach will cause performance loss especiallyfor low C/I ratio applications. The cause of the performance loss is theself-interference between the MIMO channels that is not easy to reducein time domain.

SUMMARY OF THE INVENTION

[0017] One broad aspect of the invention provides a MIMO-OFDMtransmitter adapted to transmit a header symbol format in whichsub-carriers of a header OFDM symbol are divided into a non-contiguousset of sub-carriers for each of a plurality of antennas, with eachantenna transmitting the header OFDM symbol only on the respective setof sub-carriers.

[0018] In some embodiments, there are N antennas and a different set ofsub-carriers separated by N sub-carriers is assigned to each of theplurality of antennas.

[0019] In some embodiments, the header symbols contain a multiplexeddedicated pilot channel on dedicated pilot channel sub-carriers andcommon synchronization channel on common synchronization channelsub-carriers for each of the plurality of antennas.

[0020] In some embodiments, the header OFDM symbols further containmultiplexed broadcasting sub-carriers for each of the plurality ofantennas.

[0021] In some embodiments, the transmitter is further adapted totransmit a preamble having a prefix, followed by two identical OFDMsymbols having said header OFDM symbol format. In some embodiments, theprefix is a cyclic extension of the two identical OFDM symbols.

[0022] In some embodiments, the pilot channel has a BTS specific mappedcomplex sequence allowing efficient BTS identification.

[0023] In some embodiments, the common synchronization channel isdesigned for fast and accurate initial acquisition.

[0024] In some embodiments, the common synchronization channel is usedfor course synchronization and fine synchronization and the pilotchannel is used for fine synchronization.

[0025] In some embodiments, the common synchronization channel is usedto transmit a complex sequence which is different for each transmitantenna of one transmitter, but which is common for respective transmitantennas of different transmitters within a communications network.

[0026] In some embodiments, the transmitter is further adapted totransmit OFDM frames beginning with said preamble, and having scatteredpilots throughout a remainder of the OFDM frame.

[0027] In some embodiments, during the preamble, for each of N transmitantennas, dedicated pilot channel sub-carriers are transmitted andcommon synchronization channel sub-carriers are transmitted andbroadcasting channel sub-carriers are transmitted.

[0028] In some embodiments, the sub-carriers of the preamble OFDMsymbols are organized as a repeating sequence of {dedicated pilotchannel for each of N transmit antennas, common synchronization channelsub-carrier for each of N transmit antennas} arranged in a predeterminedorder.

[0029] In some embodiments, the sub-carriers of the preamble OFDMsymbols are organized as a repeating sequence of {at least one dedicatedpilot channel sub-carrier for each of N transmit antennas, at least onecommon synchronization channel sub-carrier for each of N transmitantennas, at least one broadcast channel sub-carrier} arranged in apredetermined order.

[0030] Another broad aspect of the invention provides a MIMO-OFDMreceiver adapted to receive a header symbol format in which sub-carriersof a header OFDM symbol are divided into a non-contiguous set ofsub-carriers for each of a plurality of antennas, with each antennatransmitting the header OFDM symbol only on the respective set ofsub-carriers.

[0031] In some embodiments, the receiver is adapted to receive from Ntransmit antennas with a different set of sub-carriers separated by Nsub-carriers assigned to each of the plurality of transmit antennas.

[0032] In some embodiments, the receiver is further adapted to performfine synchronization on the basis of the common synchronization channelsub-carriers and/or the dedicated pilot channel sub-carriers.

[0033] Another broad aspect of the invention provides a transmitteradapted to transmit a packet data frame structure. The packet data framestructure has a superframe having a length corresponding to asynchronization period of a network; the superframe containing aplurality of radio frames; each radio frame containing a plurality ofTPS (transmission parameter signalling) frames corresponding to anadaptive coding and modulation period; each TPS frame containing aplurality of slots corresponding to an air interface slot size; eachslot containing a plurality of OFDM symbols, with the first two symbolsof the first slot of the first TPS frame of each OFDM frame being usedas header OFDM symbols.

[0034] In some embodiments, the header OFDM symbols have a header OFDMsymbol format in which sub-carriers of a header OFDM symbol are dividedinto a non-contiguous set of sub-carriers for each of a plurality ofantennas, with each antenna transmitting the header OFDM symbol only onthe respective set of sub-carriers.

[0035] In some embodiments, the header OFDM symbols contain multiplexedpilot channel sub-carriers and common synchronization channelsub-carriers for each of the plurality of antennas.

[0036] In some embodiments, the header OFDM symbols further containmultiplexed broadcasting channel sub-carriers for each of the pluralityof antennas.

[0037] In some embodiments, the transmitter is further adapted totransmit in a plurality of different modes by transmitting a differentnumber of OFDM symbols per slot with an unchanged slot duration and withno change to the frame structure above the slot.

[0038] In some embodiments, wherein modes with an increased number ofOFDM symbols per slot are realized by shortening OFDM symbol duration,and shortening FFT size, but not changing sampling frequency.

[0039] In some embodiments, the transmitter is further adapted totransmit to a respective set of users for each TPS frame and to signalfor each TPS frame which users should demodulate the entire TPS frame.

[0040] Another broad aspect of the invention provides a method ofperforming synchronization at an OFDM receiver. The method involves, ateach of at least one receive antenna, sampling a received signal toproduce a respective set of time domain samples; determining at leastone course synchronization position; at each of the at least one receiveantenna:

[0041] a) for each of a plurality of candidate fine synchronizationpositions about one of said at least one course synchronizationposition:

[0042] i) for each receive antenna positioning an FFT window to thecandidate fine synchronization position and converting by FFT the timedomain samples into a respective set of frequency domain components;

[0043] ii) for each said at least one transmit antenna, extracting arespective received training sequence corresponding to the transmitantenna from the sets of frequency domain components;

[0044] iii) for each transmit antenna, calculating a correlation betweeneach respective received training sequence and a respective knowntransmit training sequence;

[0045] iv) combining the correlations for the at least one transmitantennas to produce an overall correlation result for each candidatesynchronization position;

[0046] b) determining a fine synchronization position from the pluralityof correlation values;

[0047] combining the fine synchronization positions from the at leastone receive antenna in an overall fine synchronization position.

[0048] In some embodiments, a course synchronization position isdetermined for each receive antenna and used for determining therespective fine synchronization position.

[0049] In some embodiments, a course synchronization position isdetermined for each receive antenna and an earliest of the positions isused determining the fine synchronization positions for all receiveantennas.

[0050] In some embodiments, the course synchronization position isdetermined in the time domain for at least one receive antenna bylooking for a correlation peak between the time domain samples over twoOFDM symbol durations.

[0051] In some embodiments, the method is applied at an OFDM receiverhaving at least two antennas, and combining the fine synchronizationpositions from the at least one receive antenna in an overall finesynchronization position comprises selecting an earliest of the finesynchronization positions.

[0052] In some embodiments, sampling a received signal to produce a setof time domain samples is done for at least three OFDM symbol durations;determining at least one course synchronization position comprisesperforming a course synchronization in the time domain by looking for acorrelation peak between the time domain samples received over two OFDMsymbol durations to identify a course synchronization position by:

[0053] a) calculating a plurality of correlation values, eachcorrelation value being a correlation calculated between a first set oftime domain samples received during a first period having one OFDMsymbol duration and a second set of time domain samples received duringa second period immediately following the first period and having OFDMsymbol duration, for each of a plurality of starting times for saidfirst period;

[0054] b) identifying the course synchronization position to be amaximum in said plurality of correlation values.

[0055] In some embodiments, combining the correlations for the at leastone transmit antennas to produce an overall correlation result for eachcandidate synchronization position comprises multiplying together thecorrelations for the at least one transmit antenna for each candidatesynchronization position.

[0056] In some embodiments, the method is applied to a single transmitantenna single receive antenna system.

[0057] In some embodiments, the training sequence is received on commonsynchronization channel sub-carriers.

[0058] In some embodiments, the training sequence is received during anOFDM frame preamble.

[0059] In some embodiments, the training sequence is received ondedicated pilot channel sub-carriers.

[0060] In some embodiments, the training sequence is received during anOFDM frame preamble.

[0061] Another broad aspect of the invention provides an OFDM receiverhaving at least one receive antenna; for each said at least one receiveantenna, receive circuitry adapted to sample a received signal toproduce a respective set of time domain samples; a course synchronizeradapted to determine at least one course synchronization position; afine synchronizer comprising at least one FFT, at least one correlatorand at least one combiner, adapted to, at each of the at least onereceive antenna:

[0062] a) for each of a plurality of candidate fine synchronizationpositions about one of said at least one course synchronizationposition:

[0063] i) for each receive antenna position an FFT window to thecandidate fine synchronization position and convert by FFT the timedomain samples into a respective set of frequency domain components;

[0064] ii) for each said at least one transmit antenna, extract arespective received training sequence corresponding to the transmitantenna from the sets of frequency domain components;

[0065] iii) for each transmit antenna, calculate a correlation betweeneach respective received training sequence and a respective knowntransmit training sequence;

[0066] iv) combine the correlations for the at least one transmitantennas to produce an overall correlation result for each candidatesynchronization position;

[0067] b) determine a fine synchronization position from the pluralityof correlation values;

[0068] the receiver being further adapted to combine the finesynchronization positions from the at least one receive antenna in anoverall fine synchronization position.

[0069] In some embodiments, the receiver has at least two receiveantennas, and is adapted to combine the fine synchronization positionsfrom the at least one receive antenna in an overall fine synchronizationposition by selecting an earliest of the fine synchronization positions.

[0070] In some embodiments, the receiver is adapted to combine thecorrelations for the at least one transmit antennas to produce anoverall correlation result for each candidate synchronization positionby multiplying together the correlations for the at least one transmitantenna for each candidate synchronization position.

[0071] In some embodiments, the receiver is adapted to receive thetraining sequence on common synchronization channel sub-carriers.

[0072] In some embodiments, the receiver is adapted to receive thetraining sequence on dedicated pilot channel sub-carriers.

[0073] Another broad aspect of the invention provides a method ofperforming fine synchronization. The method involves, at each at leastone receive antenna receiving OFDM symbols containing a respectivereceived frequency domain training sequence for each of at least onetransmit antenna; performing fine synchronization in the frequencydomain by looking for maximum correlations between known frequencydomain training sequences and the received frequency domain trainingsequences.

[0074] Another broad aspect of the invention provides a method oftransmitting signals enabling fine synchronization. The method involvesfrom each of at least one transmit antenna, transmitting OFDM symbolscontaining a respective frequency domain training sequence.

[0075] In some embodiments, a different frequency domain trainingsequence is transmitted by each transmit antenna, but the same frequencydomain training sequence is transmitted by corresponding antenna ofother transmitters.

[0076] Another broad aspect of the invention provides a method ofperforming cell selection at an OFDM receiver. The method involves ateach of at least one receive antenna, sampling a received signal toproduce a respective set of time domain samples; determining at leastone course synchronization position; at each of the at least one receiveantenna:

[0077] a) performing a frequency domain correlation between at least onereceived common synchronization sequence extracted from commonsynchronization channel sub-carriers in the received signal and acorresponding common synchronization sequence of a respective pluralityof transmit antennas to identify a plurality of candidate correlationpeaks;

[0078] b) selecting the M strongest correlation peaks for furtherprocessing;

[0079] c) at each correlation peak, reconverting time domain samplesinto frequency domain components and processing pilot channelsub-carriers, these containing transmitter specific information, toidentify a transmitter associated with each correlation peak;

[0080] d) determining a C/I or similar value for each transmitter thusidentified;

[0081] selecting the transmitter having the largest C/I determined forany of the at least one receive antenna.

[0082] In some embodiments, performing a frequency domain correlationbetween at least one received common synchronization sequence extractedfrom common synchronization channel sub-carriers in the received signaland a corresponding common synchronization sequence of a respectiveplurality of transmit antennas to identify a plurality of candidatecorrelation peaks comprises:

[0083] a) for each of a plurality of candidate fine synchronizationpositions about one of said at least one course synchronizationposition:

[0084] i) for each receive antenna positioning an FFT window to thecandidate fine synchronization position and converting by FFT the timedomain samples into a respective set of frequency domain components;

[0085] ii) for each of at least one common synchronization sequence,each common synchronization sequence having been transmitted by atransmit antenna of each of at least one transmitter, extracting arespective received training sequence corresponding to the transmitantennas from the sets of frequency domain components;

[0086] iii) for each of the at least one common synchronizationsequence, calculating a correlation between each respective receivedcommon synchronization sequence and a respective known commonsynchronization sequence;

[0087] iv) combining the correlations to produce an overall correlationresult for each candidate synchronization position;

[0088] b) determining at least one peak in the correlations, each saidat least one peak being local maxima in the correlations.

[0089] In some embodiments, the method further involves reconvertingtime domain samples into frequency domain components based on the finesynchronization position of the selected transmitter and performing afurther fine synchronization based on a dedicated pilot channel for thattransmitter.

[0090] In some embodiments, the method is applied to a MIMO-OFDM frameformat having a header symbol format in which subcarriers of a headersymbol are divided into a non-contiguous set of subcarriers for each ofa plurality of antennas, with each antenna transmitting header symbolsonly on the respective set of sub-carriers, and wherein the headersymbols contain multiplexed pilot channel sub-carriers and commonsynchronization channel sub-carriers for each of the plurality ofantennas, the frame beginning with two identical header OFDM symbolsduring which contents of the pilot channel sub-carriers are repeated andcontents of the synchronization channel sub-carriers are repeated, thecommon synchronization channel sub-carriers carrying a complex sequencewhich is different for respective antenna of one base station and beingcommon across multiple base stations, and contents of the dedicatedpilot channel sub-carriers being at least locally unique to a particularbase station.

[0091] In some embodiments, the method further involves for transmitterswitching, averaging the C/I or similar value over a time interval foreach transmitter thus identified, and at the end of the time intervalinstigating a transmitter switch to the transmitter with the largestaverage C/I or similar value if different from a currently selectedtransmitted.

BRIEF DESCRIPTION OF THE DRAWINGS

[0092] Preferred embodiments of the invention will now be described ingreater detail with reference to the accompanying diagrams, in which:

[0093]FIG. 1A is the frame structure of IEEE 802.11 standard in the timedomain;

[0094]FIG. 1B is the frame structure of FIG. 1A in the frequency domain;

[0095]FIG. 2A is a packet data frame structure provided by an embodimentof the invention;

[0096]FIG. 2B is a packet frame hierarchy provided by an embodiment ofthe invention;

[0097]FIG. 3 is a proposed header structure provided by an embodiment ofthe invention;

[0098]FIG. 4 is a preamble header structure in the time domain providedby an embodiment of the invention;

[0099]FIG. 5 is a preamble header structure in the frequency domainprovided by an embodiment of the invention;

[0100]FIG. 6 is a conceptual schematic view of a MIMO-OFDM transmitterprovided by an embodiment of the invention;

[0101]FIG. 7A is a block diagram of a MIMO-OFDM course synchronizationfunctionality;

[0102]FIG. 7B is a block diagram of a MIMO-OFDM fine synchronizationfunctionality;

[0103]FIG. 8 is a plot of a signature sequence correlation output forpilot channel showing several candidate synchronization position;

[0104]FIG. 9 is a plot of a BTS identification simulation; and

[0105]FIG. 10 is a flowchart of a method for cell selection andre-selection for MIMO-OFDM provided by an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0106] Referring now to FIG. 2A, an OFDM packet frame structure providedby an embodiment of the invention is shown. Transmit OFDM symbol streamsare organised into such frames. Each frame consists of three majorcomponents: preamble 300, scattered pilots 302, and traffic data symbols304. The insertion of the preamble allows UE (user equipment) to performthe following fundamental operations: fast BTS (base station) access,BTS identification and C/I ratio measurement, framing and timingsynchronization, frequency and sampling clock offset estimation andinitial channel estimation. The design of a frame preamble withminimized overhead is critical to maximum spectral efficiency and radiocapacity.

[0107] Referring now to FIG. 2B, a frame hierarchy for MIMO-OFDM isorganized according to an embodiment of the invention as follows: at thehighest level are OFDM superframes 500 (two shown). The duration of thesuperframe is determined by the network synchronization period (forexample 1-second). The superframe is composed of several 10 ms radioframes 502 also referred to as OFDM frames. There would be 100 10 msOFDM frames 502 in a 1 s superframe 500.

[0108] To support adaptive coding modulation (ACM), a fast signallingchannel (TPS channel-transmission parameter signalling) is introduced.Each OFDM frame 502 is subdivided into TPS frames 504, in theillustrated example there are five 2 ms TPS frames for each 10 ms radioframe 502. The frame length used for TPS in some embodiments is the sameas the duration of the ACM unit. Each TPS frame also contains signallinginformation which allows each user to determine whether the current TPSframe contains data for them or not. A TPS frame may contain data formultiple users.

[0109] The TPS frame 504 can be divided further into several slots 506,each of which consists of several OFDM symbols. In the illustratedexample, each TPS frame 504 is subdivided into 3 slots 506. The durationof the slot 506 depends upon the air interface slot size. The smallesttransmission unit is one OFDM symbol 508, 510. The duration of one OFDMsymbol is determined by the transmission environment characteristics,for example, the maximum channel delay, the system-sampling clock andthe maximum Doppler. In the illustrated example, there are four OFDMsymbols 508, 510 per slot 506.

[0110] To reduce the overhead caused by the insertion of the guardinterval between OFDM symbols, different OFDM symbol modes each with adifferent symbol duration and a different prefix can be designed, forexample, 0.5 k mode and 1 k mode. To simplify the system the samplingfrequency is kept unchanged when doing the mode switching. Thesedifferent modes are described in more detail below.

[0111] The frame structure of FIG. 2B gives an example of a framestructure hierarchy compatible to the UMTS air-interface. At the OFDMsymbol level, there are two different types of OFDM symbols. Theseinclude the preamble OFDM symbols 508 and regular data symbols 510.

[0112] Referring now to FIG. 4, which is a time domain representation,each OFDM frame starts with a preamble, which consists of severalidentical header OFDM symbols 603, 605 preceded by a prefix 607 which isa cyclic extension of the header OFDM symbols. A repetition structure isused to assist synchronization. By performing a correlation betweenadjacent OFDM symbols until two identical symbols are identified, thestart of an OFDM frame can be found. By way of example, there may be1056 samples used per OFDM symbol. For the preamble, during the prefix607, the last 64 samples of the header OFDM symbols are transmitted.There is no prefix for the second header OFDM symbol. The header isinserted periodically, and for the example of FIG. 2B, this occurs every10 ms, i.e. at the beginning of every OFDM frame.

[0113] Referring again to FIG. 2B, it is noted that for non-header OFDMsymbols, i.e. for the regular OFDM symbols 510, every OFDM symbolpreferably also has a prefix. In “1K” mode, there are 32 prefix samples,and 1024 actual samples representing the FFT size, for a total of 1056samples per symbol. In 1/2K mode, there is a 16 sample prefix, and then512 samples per symbol (representing the FFT size) for a total of 528samples/symbol. Advantageously, using the frame structure of FIG. 2Bthese different modes can be supported without changing the samplingfrequency. When in 1/2K mode, there are twice as many OFDM symbols 510per slot 506. The particular mode chosen at a given instant should besuch that the prefix size is greater than the maximum channel delay. in1/K mode, more OFDM symbols are sent with fewer sub-carriers. This ismore robust to high Doppler, because the symbol duration is shorter.Also, the spacing between the sub-carriers is larger further enhancingtolerance to Doppler. Thus, there is a unified frame structure whichaccommodates different FFT sizes, but with the same sampling rate a thereceiver. Preferably the same preamble is used even for the differentmodes.

[0114] OFDM is a parallel transmission technology. The whole usefulbandwidth is divided into many sub-carriers, and each sub-carrier ismodulated independently. According to an embodiment of the invention, toseparate different antenna with multiple antennas transmission, duringthe header not all sub-carriers are used on all transmit antennas.Rather, the sub-carriers are divided between antennas. An example ofthis will now be described with reference to FIG. 3. The sub-carrierfrequencies contained within an OFDM symbol are each represented bycircles. In this example it is assumed that there are two transmittingantennas in the MIMO system. FIG. 3 shows OFDM symbols with the varioussub-carriers spaced along the frequency axis 400, and with the contentsof all the sub-carriers at a given instant representing one symbol intime, as indicated along the time axis 402. In this case, the first twoOFDM symbols 408, 410 are used for dedicated pilot channel informationwhile the remaining symbols (only two shown, 412, 414) are used forregular OFDM symbols. The dedicated pilot channel informationtransmitted on the first two OFDM symbols 408, 410 alternates bysub-carrier between being transmitted by the first antenna and thesecond antenna. This is indicated for the first sub-carrier 404 which istransmitting dedicated pilot channel information for the firsttransmitter and sub-carrier 406 which is transmitting dedicated pilotchannel information for the second sub-carrier, and this pattern thenrepeats for the remainder of the sub-carriers. The other OFDM symbols412, 414 contain information transmitted by both antennas. It is to beunderstood that other spacings could alternatively be used. Furthermore,if there are more then two transmit antennas, the pilot channelinformation would then alternate by sub-carrier in some predeterminedpattern between all of the transmit antennas.

[0115] In another embodiment, a common synchronization channel, anddedicated pilot channel are frequency multiplexed onto the headersymbols. A respective set of non-overlapping sub-carriers are assignedfor each antenna to transmit respective dedicated pilot channel andcommon synchronization channel.

[0116] In another embodiment a common synchronization channel, dedicatedpilot channel and a broadcasting channel are frequency multiplexed ontothe header symbols. Under this arrangement, the total usefulsub-carriers of the header symbols are separated into three groups.These three groups are mapped onto the common synchronization channel,dedicated pilot channel and the broadcasting channel respectively.

[0117] An example of the mapping of the different channels in theMIMO-OFDM system with two-transmitter diversity is shown in FIG. 5. Inthis example, there are shown four OFDM symbols 712, 714, 716, 718 twoof which 712, 714 are header symbols. During the header symbols 712, 714every second sub-carrier is used for the first antenna with theremaining sub-carriers used for the second antenna. This is easilygeneralized to higher numbers of antennas. For this example, it isassumed that there are two transmit antennas in the MIMO system. Everysixth sub-carrier starting at the first sub-carrier 700 is for the firsttransmitter dedicated pilot channel sub-carriers. Every sixthsub-carrier starting at the second sub-carrier 702 is for the secondtransmitter dedicated pilot channel sub-carrier. Every sixth sub-carrierstarting at the third sub-carrier 704 is for the first transmittercommon synchronization channel sub-carrier. Every sixth sub-carrierstarting at the fourth sub-carrier 706 is for the second transmittercommon synchronization channel sub-carrier. Every sixth sub-carrierstarting at the fifth sub-carrier is for broadcasting channelsub-carriers for the first antenna, and every sixth sub-carrier startingat the sixth sub-carrier 710 is for broadcasting channel sub-carriersfor the second antenna.

[0118] The common synchronization channel is a universal channel forinitial access. It can also be used for synchronization and preliminarychannel estimation. The different transmitters share the commonsynchronization sub-carriers when transmitter diversity is applied. Inwhich case as indicated above the common synchronization channel isdivided between different transmitters. A common complex sequence knownby all the terminals is used to modulate the sub-carriers reserved forthe common synchronization channel. The same common synchronizationsequence is transmitted by all base stations within a system. There maybe one or more such synchronization sequences in the event that thereare multiple transmit antennas such that each transmit antenna cantransmit a unique synchronization sequence. Using the synchronizationsequence, mobile stations are able to find initial synchronizationpositions for further BTS identification by looking for a correlationpeak between received synchronization sequence and the known transmittedsynchronization sequence.

[0119] The dedicated pilot channel is used for BTS/cell identification,and supports C/I measurement for the cell selection, cell switching andhandoff. A unique complex sequence, for example a PN code, is assignedto each BTS and used to modulate the dedicated pilot sub-carriers. Adifferent unique sequence is transmitted by each antenna in the multipletransmit antenna case. Unlike the case for the common synchronizationchannel, different base stations transmit using different pilotsequences. The quasi-orthogonality of the PN codes assigned to differentBTSs makes it possible to do access point identification and initialinterference measurement. The dedicated pilot channel can also be usedto assist the synchronization processing.

[0120] To fully utilize the sub-carriers in the header OFDM symbols, asindicated above, some sub-carriers are preferably used as a broadcastingchannel. In the example of FIG. 5, two of every six sub-carriers areused for this purpose. The broadcasting channel can carry importantsystem information. STTD (space time transmit diversity) schemes cannotbe used for the broadcasting channel (or any of the sub-carriers in theheader OFDM symbols) because of it will destroy the repetition structureof the header OFDM symbols which is required by synchronizationalgorithms. However transmitting the broadcasting information by alltransmitters on the same sub-carrier may cause destructive interferencebetween transmitters. To solve that problem the broadcasting channel ispartitioned between different transmitters, so in the two transmitantenna case, the sub-carriers (mapped for the broadcasting channel) canbe assigned alternatively for the transmit antenna to provide diversity.Power boosting may be applied to further enhance the broadcastingchannel.

[0121] The broadcasting information from different BTS's can bedifferent. In some embodiments broadcasting information is protected sothose users close to the cell boundaries can receive it correctly in thepresence of strong interference. A short PN code could be used to spreadthe broadcasting information. The neighbouring BTS is assigned to usedifferent code. The insertion of the broadcasting channel reduces thepreamble overhead and increases the spectrum efficiency.

[0122] The broadcast channel is used to transmit information unique tothe particular base station. A single broadcast message may be sent onthe combined broadcast channel carriers for the two antennas. Bydesigning the preamble header symbol to consist of pilot channel,synchronization channel and the broadcasting channel, the preambleheader overhead is reduced. The common synchronization channel isdesigned for fast and accurate initial acquisition. The dedicated pilotchannel with a BTS specific mapped signature allows an efficient BTSidentification. The combined common synchronization channel and thepilot channel are used together for MIMO channel estimation. The use ofthe combined common synchronization channel and the dedicated pilotchannel also allows for high accuracy synchronization. Frequency domaintraining symbols are robust to timing error and multipath environments.The preamble design allows the flexibility of the user equipment toimplement more efficient algorithms.

[0123] It is noted that the specific breakdown of sub-carriers betweenthe dedicated pilot channel in one embodiment, between the dedicatedpilot channel and common synchronization channel in another embodiment,and between the dedicated pilot channel, common synchronization channeland broadcast channels in another embodiment, are only specificexamples. These can be allocated in any suitable manner.

[0124] Referring now to FIG. 6, shown is a conceptual schematic of aMIMO-OFDM transmitter 10. A first sample set of four OFDM symbols 201 isshown transmitted from a first transmit antenna 21 and a second sampleset of four OFDM symbols 203 is shown transmitted from a second transmitantenna 23. In general an OFDM transmitter will have N_(ant) transmitantennae, where N_(ant) is a design parameter. Within the MIMO-OFDMtransmitter 10, data originating from a demultiplexer 23 are sent to oneof either a first OFDM component 24 connected to transmit antenna 21 ora second OFDM component 26 connected to transmit antenna 23. Thecomponents organize the data onto sub-carriers of OFDM symbols and OFDMframes, each sub-carrier being at a different orthogonal frequency. EachOFDM component 24, 26 has a respective header inserter 29 which insertsheader OFDM symbols. The sample sets of OFDM symbols 201 and 203represent the first four OFDM symbols of the transmitted OFDM frame fromtransmit antennae 21 and 23, respectively, where each row of datasymbols or pilot symbols is an OFDM symbol. A first OFDM symbol 13 and asecond (identical to the first) OFDM symbol 14 represent the two headerOFDM symbols unique to the OFDM frame transmitted by first transmitantenna 21. Similarly, a third OFDM symbol 17 and a fourth (identical tothe third) OFDM symbol represent the two header OFDM symbols unique tothe OFDM frame transmitted by the second transmit antenna 23. Four OFDMsymbols 15, 16, 19, 20 are typically non-identical OFDM symbols made upof a plurality of data symbols, with at least one data symbol indicatedgenerally at 11 on each OFDM sub-carrier. An entire OFDM frame wouldtypically have many more data symbols. Also, the OFDM symbols 201 aretransmitted concurrently, and with the same timing, as OFDM symbols 203.

[0125] In this example, the two identical header OFDM symbols consist ofdedicated pilot channel sub-carriers 12 and common synchronizationchannel sub-carriers 9. There may also be broadcast channelsub-carriers, not shown. The dedicated pilot channel sub-carriers areused for C/I ratio measurement and BTS identification and finesynchronization as detailed below; they can also be used for initialchannel estimation. The common synchronization channel sub-carriers 9are used for course synchronization and fine synchronization, initialaccess, and initial channel estimation.

[0126] In the illustrated example, during the two header OFDM symbols,the first of every four consecutive sub-carriers is used to carrydedicated pilot channel symbols transmitted by transmitting antenna 21.Similarly, the second of every four consecutive sub-carriers is used tocarry dedicated pilot channel symbols transmitted by transmittingantenna 23.

[0127] The dedicated pilot channel symbols transmitted on the pilotchannel sub-carriers 12, 25 are defined by base station/sector specificPN sequence. A set of symbols from a complex pseudo-random PN sequenceunique to the base station is mapped onto the dedicated pilot channelsub-carrier locations in the header OFDM symbols.

[0128] The third of every four consecutive sub-carriers in the twoheader symbols is used to carry common synchronization channel symbolstransmitted by transmitting antenna 21. Similarly the fourth of everyfour consecutive sub-carriers is used to common synchronization channelsymbols transmitted by transmitting antenna 23.

[0129] The common synchronization channel symbols transmitted on thecommon synchronization sub-carriers 9, 27 are defined by unique complexpseudo-random PN sequence for each transmit antenna 21 and 23. A set ofsymbols from this complex pseudo-random PN sequence is mapped onto thecommon synchronization channel sub-carriers in the header OFDM symbols.That is, the common synchronization channel symbols of each frametransmitted through each transmitting antenna use a PN code unique tothat transmitting antenna but which is the same for correspondingtransmitting antennas of other base stations. In the present examplePN_(SYNC) ⁽¹⁾ is associated with transmit antenna 21 and PN_(SYNC) ⁽²⁾is associated with transmit antenna 23. However, similar antennae indifferent transmitters throughout the communication network will use thesame PN code. For example, the common synchronization channel symbolsfor a first transmit antenna 21 on all transmitters within the networkwill use one PN code (PN_(SYNC) ^((l))) and the common synchronizationchannel symbols for a second transmit antenna 22 on all transmitterswithin the network will use a different PN code (PN_(SYNC) ⁽²⁾).

[0130] Referring to FIG. 7A, a block diagram of MIMO-OFDM receiverfunctionality is shown which is adapted to perform coarsesynchronization based on the two repeated OFDM header symbolstransmitted by each transmit antenna as detailed above. The OFDMreceiver includes a first receiving antenna 734 and a second receivingantenna 735 (although more generally there will be a plurality of Nreceiving antennae). The first receiving antenna 734 receives a firstreceived signal at RF receiver 736. The first received signal is acombination of the two signals transmitted by the two transmittingantennae 21 and 23 of FIG. 6, although each of the two signals will havebeen altered by a respective channel between the respective transmittingantenna and the first receiving antenna 734. The second receivingantenna 735 receives a second received signal at RF receiver 739. Thesecond received signal is a combination of the two signals transmittedby the two transmitting antennae 21 and 23, although each of the twosignals will have been altered by a respective channel between therespective transmitting antenna and the second receiving antenna 735.The four channels (between each of the two transmitting antennae andeach of the two receiving antennae) may vary with time and withfrequency, and will in general be different from each other.

[0131] Coarse synchronization is performed for the first receive antenna734 by a coarse synchronizer 737 on discrete time samples of a receivedsignal to determine an approximate range of a location of the startingposition of the first header symbol. A similar process is performed bycourse synchronizer 741 for the second antenna 735. Coarsesynchronization is facilitated by the use of repeated header symbols atthe OFDM transmitter. The coarse synchronizer 737 performs correlationmeasurements on time domain signal samples in successive OFDM symbols.The time domain signal sample yielding the highest correlationmeasurement is the coarse synchronization position n_(coarse). Thecourse synchronization position n_(coarse) is then used as the positionon which to locate an FFT window within the FFT functions used in finesynchronization.

[0132] Initially, the coarse synchronizer 737 starts the time domaincoarse synchronization processing. A running buffer (not shown) is usedto buffer discrete time samples of the received signal over threesuccessive OFDM symbol period, and then calculates the auto-correlationγ_(t)(n) between samples collected during two successive OFDM symboldurations as follows:${\gamma_{t}(n)} = {\sum\limits_{i = 0}^{{Nheader} - 1}\quad {{x\left( {n + i} \right)} \cdot {x^{*}\left( {n + i + N_{header}} \right)}}}$

[0133] where x(n) is the time domain samples of the received signal,N_(header) is the number of samples taken over one OFDM symbol duration.

[0134] In some embodiments, a moving correlator is applied in the realtime implementation to save calculation power.

[0135] In one embodiment, the values of γ_(t)(n) are calculated insequence, for n=1 (until n=N_(header)), until a correlation value isabove a threshold, after which a maximum search is enabled. Thecomputation of the correlation values continues and the maximum searchprocess will continue until the correlation result is below thethreshold again. The sample position corresponding to the maximumcorrelation value is the coarse synchronization position:

n _(coarse)=argmax(|γ_(t)(n)|)nε{γ _(t)(n)>γ_(threshold})

[0136] The threshold is typically calculated from the averageauto-correlation values within one frame. Alternatively, another way offinding the maximum is to determine a local maximum for each OFDM symbolover an OFDM frame which might be 60 symbols in length for example.Then, the overall maximum is taken to be the maximum of the localmaxima. This process is conducted both course synchronizers. In theevent fine synchronization is to proceed jointly, the overall coursesynchronization position may be taken as some combination of the twosynchronization values, and is preferably taken to be the earlier of twocourse synchronization positions thus determined. Alternatively, eachfine synchronizer (detailed below) can work from a respective coursesynchronization position.

[0137] Referring to FIG. 7B, a block diagram is shown of an MIMO-OFDMfine synchronization functionality is shown. In one embodiment, the finesynchronization functionality is adapted to perform fine synchronizationbased on the two-repeated OFDM header symbols transmitted by eachtransmit antenna as detailed above using the common synchronizationchannel and/or the dedicated pilot channel. More generally, the finesynchronization functionality can perform fine synchronization for OFDMframes within which some known training sequence has been embedded.Also, an input to the fine synchronization process is a coursesynchronization position. This course synchronization position may bedetermined using the above discussed method, or using any other suitablemethod. The components which are identical to those of FIG. 7A aresimilarly numbered and in an actual implementation would be shared ifthe common synchronizers of FIG. 7A are to be used. The functionality ofFIG. 7B is replicated for each of the one or more receive antenna.

[0138] A fine synchronization process is performed for each of one ormore receive antennae, and then an overall synchronization position istaken based on a combination of the fine synchronization positions. Byway of overview, once the coarse synchronizers have determined thecoarse synchronization position(s) n_(coarse), each fine synchronizerperforms an FFT on the signal samples on either side of the coarsesynchronization position, to generate frequency domain components overthe frequency band of OFDM sub-carriers. Each fine synchronizer searchesthe frequency domain components in order to locate the precise locationof the FFT window. The precise location of the FFT window is required inorder to perform OFDM demodulation in the frequency domain. The finesynchronizer locates the precise location of the FFT window byperforming correlation measurements between the known PN codes(PN_(SYNC) ⁽¹⁾& PN_(SYNC) ⁽²⁾) and the frequency components within asearching window defined with respect to the coarse synchronizationposition n_(coarse). The correlation measurements performed by each finesynchronizer are performed in the frequency domain, and one set ofcorrelation measurements is performed for each known PN code (PN_(SYNC)⁽¹⁾ & PN_(SYNC) ⁽²⁾) that is, for each transmitting antenna 21 and 23(or for how many of the one or more transmit antenna there are).

[0139] Each fine synchronizer selects N_(symbol) signal samples startingat an initial signal sample within the searching window, whereN_(symbol) is the number of signal samples in an OFDM symbol. For eachtransmitting antenna, each fine synchronizer determines a correlationmeasurement between the frequency domain signal samples and the PN codecorresponding to the transmitting antenna.

[0140] More specifically, fine synchronization searching is performednear n_(coarse). Supposing that the searching window is 2N+1, thesearching range is from (n_(coarse)−N) to (n_(coarse)+N). Letn_(start)(i)=n_(coarse)+N−i represent the sample index within the finesearching window, where i=0 , . . . , 2N. The fine synchronizationstarts from i=0. Then N_(symbol) samples are taken starting fromn_(start) (0), the prefix is removed and FFT is performed. The receivedOFDM symbol in frequency domain can be written as:

R(l,i)=FFT(x(n(i),l)), n(i)=[n _(start)(i)+N _(prefix) , n _(start)(i)+N_(symbol)−1]; l=1, . . . N _(FFT);

[0141] where N_(prefix) is the number of prefix samples and N_(FFT) isthe FFT size.

[0142] From R, the complex data R^((j,k)) _(SYNC) carried by the commonsynchronization channel of different transmitters is extracted, sincecommon synchronization channels are divided between differenttransmitters in MIMO OFDM system. More generally, the complex the datacorresponding to a transmitted training sequence is extracted. Thecorrelation between R^((j,k)) _(SYNC) and PN*^((j)) _(SYNC) is:${{\gamma_{f}^{({j,k})}(i)} = {\sum\limits_{m = 0}^{N_{SYNC} - 1}\quad {{R_{SYNC}^{({j,k})}\left( {m,i} \right)} \cdot {{PN}_{SYNC}^{*{(j)}}(m)}}}},{i = 0},\ldots \quad,{2N}$

[0143] where j=1, 2, . . . , N_(Tx) indicates transmitter, k=1, 2, . . ., N_(Rx) indicates receiver, PN^((j)) _(SYNC) is the common SYNC PN codefor j^(th) transmitter and N_(SYNC) is the size of common PN code.

[0144] Then the starting point index n_(start) is shifted by one(n_(start)(1)=n_(start)(0)−1), and another N_(symbol) samples areprocessed as described above. In order to get the new frequency domaindata R^((j,k)) _(SYNC) (m,i), we need to perform FFT again. An iterativemethod can be used for this purpose to reduce the computationalcomplexity:

R(l,i)=R(l,i−1).e ^(i2π(k−1)/NFFT) +x(n _(start)(i)+N _(prefix))−x(n_(start)(i−1)+N _(symbol)−1)

[0145] where NFFT is the FFT size. Extracting R^((j,k)) _(SYNC) (m,i),the new correlation is calculated. The above procedure is continueduntil n_(start) moves out of the fine searching window.

[0146] For each$n_{fine} = {\arg \quad {\max \left( {\prod\limits_{j = 1}^{N_{Tx}}\quad {\prod\limits_{l = 1}^{N_{Rx}}\quad {{\gamma_{f}^{({j,k})}(i)}}}} \right)}}$

[0147] receive antenna, a respective fine synchronization position canbe found by finding n_(start)(i) corresponding to the maximum of theproducts of the correlation results from different antennas over i=0, .. . , 2N. In mathematical terms, for the kth receive antenna, arespective fine synchronization position can be selected according to:${n_{fine}(k)} = {\arg \quad {\max \left( {\prod\limits_{j = 1}^{N_{Tx}}\quad {{\gamma^{({j,k})}(i)}}} \right)}}$

[0148] To reduce the possibility of false alarm, a criterion may be set.For example, the fine synchronization may be considered to be achievedif the following condition is satisfied,${\max \left( {\prod\limits_{j = 1}^{N_{Tx}}\quad {{\gamma^{({j,j})}(i)}}} \right)} > {N_{threshold} \cdot \frac{1}{{2N} + 1} \cdot {\sum\limits_{i = 0}^{2N}\quad {\prod\limits_{j = 1}^{N_{Tx}}\quad {{\gamma^{({j,j})}(i)}}}}}$

[0149] where N_(threshold) is a factor determined by the pre-set finesearching window size. Preferably, an overall fine synchronizationposition is then taken to be the earliest of the fine synchronizationpositions determined for the different receive antennas.

[0150] The fine synchronization process for one receive antenna isillustrated diagrammatically in FIG. 7B. At the output of the firstreceiver 736, blocks D0 738 through D2N 742 represent alignment of theFFT blocks 744, . . . , 748 for the various candidate finesynchronization positions (2N+1 in all). The FFT blocks 774, . . . , 748compute an FFT on each respective set of samples. Each FFT output is fedto a correlator block for each transmit antenna. If there are twotransmit antennae, then there would be two such correlator blocks perFFT output. For example FFT 744 has an output fed to a first correlatorblock 745 for the first transmit antenna, and fed to a second correlatorblock 755 for a second transmit antenna. It is noted that if the spacingof the sub-carriers used to transmit the training sequence (the commonsynchronization sequence or pilot channel sequence in the aboveexamples), a full FFT does not need to be completed in order to recoverthe training sequence components. The correlator block 745 for the firstantenna multiplies with multiplier 747 the recovered training sequencesymbol locations of the FFT output by the known training sequence forthe first transmit antenna and these multiplications are added in summer751. This same computation done in correlator 755 for the known trainingsequence of the second transmit antenna and the training sequencelocations for the second transmit antenna. This is done at the firstreceiver for all of the different possible shifts for each transmitantenna. The correlation results across different transmit antennas foreach possible shift are multiplied together in multipliers 753. Theshift which results in the maximum of these multiplications is selectedto be the fine synchronization position for the particular receiver. Thesame process is followed for any other receive antennas, and the overallfine synchronization position is preferably taken as the earliest of thefine synchronization positions thus computed.

[0151] The timing synchronization can be tracked every frame in casethat the synchronization position drifts or losses. For example, insystems employing the previously described preamble, each time apreamble arrives at the receiver the 2-step process of synchronizationis repeated, using the same method for coarse synchronization and finesynchronization. In this case, a smaller searching window N may be usedbased on the assumption that the drift of the synchronization positionshould be around the vicinity of the current location. Afteracquisition, the dedicated pilot channel code assigned to modulatededicated pilot channels for different BTS can be used in thecorrelator, or the common synchronization sequence can be used, or someother training sequence.

[0152] An embodiment of the invention has been described with respect toan MIMO-OFDM transmitter having more than one transmitting antenna. Themethod of performing synchronization at the OFDM receiver may also beapplied to a signal received from an OFDM transmitter having only onetransmitting antenna, as long as a known training sequence is insertedin the frame by the OFDM transmitter.

[0153] Lastly, in the embodiment of the invention described thus farthere has only been one transmitter having multiple antennae and onereceiver having multiple antennae. In what follows, the concepts of theinvention will be broadened to encompass the multi-cellular environmenthaving many MIMO-OFDM transmitters and many MIMO-OFDM receivers.

[0154] Access in a Multi-Cellular Environment

[0155] System access in a multi-cellular environment introduces the newproblem of cell selection, as there will be many transmitterstransmitting the same common pilot symbols. In another embodiment of theinvention, the previously introduced transmit header is used byreceivers to perform systems access and cell selection.

[0156] During initial acquisition, the UE starts by performing coarsesynchronization. This may be done using the previously describedmethods, or some other method. After one frame duration, the coarsesynchronization position is determined. Fine synchronization searchalgorithm is performed afterwards based on the common synchronizationchannel. Because the data carried by the common synchronization channelare the same for all BTS, several fingers (peaks) can be observed in amulti-cell environment and multi-path fading propagation channels. Thesefingers usually correspond to different BTS and/or different paths.Referring to FIG. 8, shown is an example of fine synchronization (to thecommon synchronization channel) raw output computed in a multi-cellularenvironment as a function of sample index. In the present example thereare five significant fingers 400, 402, 404, 406, and 408. The Mstrongest fingers are chosen and the corresponding positions arelocated, where M is a system design parameter. These positions are usedas candidates for final synchronization and also as the positions uponwhich BTS identifications are made.

[0157] The results of FIG. 8 do not allow BTS identification because BTStransmit the same common synchronization sequences. At each candidatesynchronization position, the correlations of the received dedicatedpilot channel sub-carriers and all possible complex sequences (dedicatedpilot PN sequences) assigned to different BTS are calculated to scan forthe presence of all the possible adjacent BTSs. In the multiple transmitantenna case, preferably this correlation is done on the basis of thecombined dedicated pilot PN sequences of the multiple antennas over allof the dedicated pilot sub-carriers to generate a single correlationresult for each index. FIG. 9 shows an example of the relation betweenthe BTS scanning results and the checking points (candidatesynchronization positions). The BTS identification is realized bydetecting the PN code corresponding to the maximum correlation value ateach candidate synchronization position. C/I can be computed based onall correlation results at each checking position. At the initialacquisition stage, the cell selection is determined by selecting the BTSwith the largest C/I ratio. In the present example two BTS areidentified, a first BTS BTS1 and a second BTS BTS2. Withmultiple-antenna receiver diversity, the final decision of the cellselection should be based on the comparison of the highest C/I obtainedby different receiver antennae at a receiver.

[0158] To obtain the final synchronization position, finesynchronization is performed again, but by using the dedicated pilotchannel and the dedicated complex sequence found through the BTSidentification. A smaller searching window around the finesynchronization position is used. The final synchronization results fromdifferent receivers are compared. The position corresponding to theearliest sample in time is used as the final synchronization position.This step is to reduce the possibility that a weak path (multi-path) isselected because of the short-term fading. To reduce the false alarmprobability, a threshold is set. This threshold can be the ratio of thefinger strength with respect to the final synchronization position andthe average of the correlation within the search window.

[0159] In the normal data processing stage, the fine synchronization andthe BTS identification steps are repeated every frame when a newpreamble is received, but a small set of the candidate PN codes isapplied in the BTS scan. After BTS identification, a BTS candidates listcan be generated through searching strong interferences. This list isupdated periodically, for example every 10 ms, and provides informationfor BTS switch and even soft handoff. Certain criteria can be set inorder to trigger the BTS switch and soft handoff. To average the impactfrom the fading, the decision for BTS switching and the soft handoff maybe based on observation during a certain period. The criteria can be thecomparison of the maximum correlation values representing C and thestrongest I. It should be noted that after the cell switch and the softhandoff, the synchronization may also be adjusted by the final step inthe initial access. The overall cell selection and re-selection methodis shown in FIG. 10.

[0160] In the first step 600, coarse synchronization is performed forexample based on the preamble header in the time domain. This involvesfinding a coarse boundary between each frame by looking for twoidentical symbols. Correlating samples over adjacent symbol durationsuntil a peak is found does this. Step 600 relies on a preamble to aframe beginning with two adjacent identical symbols.

[0161] Next during step 602, at the coarse synchronization peaks, an FFTis computed, and a switch to the processing of the commonsynchronization channel in the frequency domain is made. A search windowis centered on sync position +/− a certain number of samples. The Mstrongest correlation peaks are selected, as per 604. At this time, itis not known which BTS each peak is associated with. BTS identificationhas not yet been determined.

[0162] Then at step 606, for each correlation peak, the FFT is againcomputed and the correlations associated with the fine synchronizationprocedure are computed using the dedicated pilot channels—thesecontaining a base station specific complex sequences. This isimmediately followed by step 608 where the correlation with the BTSidentification complex sequences is made in order to allow anidentification of the associated base stations. At step 610, a C/I ratiois computed for each BTS thus identified. BTS selection and BTSswitching is performed on the basis of these C/I ratios in step 612. ASindicated above, BTS switching is performed on the basis of these C/Iratios averaged over some time interval.

[0163] Finally, for access, the FFT is computed and fine synchronizationis applied to the dedicated pilot channel of the BTS with the largestC/I ratio as per step 614.

[0164] BTS initial synchronization performed on the commonsynchronization channel. A BTS specific sequence is embedded in thefrequency domain and BTS identification processing is performed in thefrequency domain allowing the elimination of MIMO-OFDM inter-channelinterference. BTS power estimation is performed based on the pilotchannel for each MIMO-OFDM BTS. BTS selection is performed based on C/Iratio measurements.

[0165] The result is improvement of the synchronization andidentification of the serving BTS in a severe multi-path channel andhigh interference environment by joint BTS synchronization and cellselection. Channel estimation may be performed on a combined commonsynchronization channel and dedicated pilot channel. Criteria areprovided for cell switching and soft handoff by C/I estimation.

[0166] In the above example, the access has been performed based on thesynchronization channel and pilot channel embedded in the previouslydiscussed preamble. More generally, the access can be performed withsuch channels embedded in any suitable manner within an OFDM symbolstream.

[0167] What has been described is merely illustrative of the applicationof the principles of the invention. Other arrangements and methods canbe implemented by those skilled in the art without departing from thespirit and scope of the present invention.

We claim:
 1. A MIMO-OFDM transmitter adapted to transmit a header symbolformat in which sub-carriers of a header OFDM symbol are divided into anon-contiguous set of sub-carriers for each of a plurality of antennas,with each antenna transmitting the header OFDM symbol only on therespective set of sub-carriers.
 2. A transmitter according to claim 1wherein there are N antennas and a different set of sub-carriersseparated by N sub-carriers is assigned to each of the plurality ofantennas.
 3. A transmitter according to claim 1 wherein the headersymbols contain a multiplexed dedicated pilot channel on dedicated pilotchannel sub-carriers and common synchronization channel on commonsynchronization channel sub-carriers for each of the plurality ofantennas.
 4. A transmitter according to claim 3 wherein the header OFDMsymbols further contain multiplexed broadcasting sub-carriers for eachof the plurality of antennas.
 5. A transmitter according to claim 1,adapted to transmit a preamble having a prefix, followed by twoidentical OFDM symbols having said header OFDM symbol format.
 6. Atransmitter according to claim 5 wherein the prefix is a cyclicextension of the two identical OFDM symbols.
 7. A transmitter accordingto claim 3 wherein the pilot channel sub-carriers have a BTS specificmapped complex sequence allowing efficient BTS identification.
 8. Atransmitter according to any one of claims 3 wherein the commonsynchronization channel is designed for fast and accurate initialacquisition.
 9. A transmitter according to claim 3 wherein the commonsynchronization channel is used for course synchronization and finesynchronization and the pilot channel is used for fine synchronization.10. A transmitter according to claim 3 wherein the commonsynchronization channel is used to transmit a complex sequence which isdifferent for each transmit antenna of one transmitter, but which iscommon for respective transmit antennas of different transmitters withina communications network.
 11. A transmitter according to claim 1 adaptedto transmit OFDM frames beginning with said preamble, and havingscattered pilots throughout a remainder of the OFDM symbols in each OFDMframe.
 12. A transmitter according to claim 1 wherein during thepreamble, for each of N transmit antennas, dedicated pilot channelsub-carriers are transmitted and common synchronization channelsub-carriers are transmitted and broadcasting channel sub-carriers aretransmitted.
 13. A transmitter according to claim 3 wherein thesub-carriers of the preamble OFDM symbols are organized as a repeatingsequence of {dedicated pilot channel for each of N transmit antennas,common synchronization channel sub-carrier for each of N transmitantennas} arranged in a predetermined order.
 14. A transmitter accordingto claim 4 wherein the sub-carriers of the preamble OFDM symbols areorganized as a repeating sequence of {at least one dedicated pilotchannel sub-carrier for each of N transmit antennas, at least one commonsynchronization channel sub-carrier for each of N transmit antennas, atleast one broadcast channel sub-carrier} arranged in a predeterminedorder.
 15. A MIMO-OFDM receiver adapted to receive a header symbolformat in which sub-carriers of a header OFDM symbol are divided into anon-contiguous set of sub-carriers for each of a plurality of antennas,with each antenna transmitting the header OFDM symbol only on therespective set of sub-carriers.
 16. A receiver according to claim 15adapted to receive from N transmit antennas with a different set ofsub-carriers separated by N sub-carriers assigned to each of theplurality of transmit antennas.
 17. A receiver according to claim 15wherein the header OFDM symbols contain multiplexed dedicated pilotchannel sub-carriers and common synchronization channel sub-carriers foreach of the plurality of transmit antennas.
 18. A receiver according toclaim 17 wherein the header OFDM symbols further contain multiplexedbroadcasting carriers for each of the plurality of antennas.
 19. Areceiver according to claim 15 adapted to receive a preamble having aprefix, followed by two identical OFDM symbols having said header OFDMsymbol format.
 20. A receiver according to claim 15 wherein thededicated pilot channel has a BTS specific mapped complex sequence, thereceiver being adapted to perform BTS identification on the basis of thededicated pilot channel.
 21. A receiver according to claim 19 whereinthe dedicated pilot channel have a BTS specific mapped complex sequence,the receiver being adapted to perform BTS identification on the basis ofthe dedicated pilot channel.
 22. A receiver according to claim 21wherein the header OFDM symbols contain multiplexed dedicated pilotchannel sub-carriers and common synchronization channel sub-carriers foreach of the plurality of transmit antennas, the receiver being furtheradapted to perform course synchronization on the common synchronizationchannel by looking for a correlation peak between consecutive OFDMsymbols which are identical.
 23. A receiver according to claim 22further adapted to perform fine synchronization on the basis of thecommon synchronization channel sub-carriers and/or the dedicated pilotchannel sub-carriers.
 24. A transmitter adapted to transmit a packetdata frame structure comprising: a superframe having a lengthcorresponding to a synchronization period of a network; the superframecontaining a plurality of radio frames; each radio frame containing aplurality of TPS (transmission parameter signalling) framescorresponding to an adaptive coding and modulation period; each TPSframe containing a plurality of slots corresponding to an air interfaceslot size; each slot containing a plurality of OFDM symbols, with thefirst two symbols of the first slot of the first TPS frame of each OFDMframe being used as header OFDM symbols.
 25. A transmitter according toclaim 24 wherein the header OFDM symbols have a header OFDM symbolformat in which sub-carriers of a header OFDM symbol are divided into anon-contiguous set of sub-carriers for each of a plurality of antennas,with each antenna transmitting the header OFDM symbol only on therespective set of sub-carriers.
 26. A transmitter according to claim 24wherein the header OFDM symbols contain multiplexed pilot channelsub-carriers and common synchronization channel sub-carriers for each ofthe plurality of antennas.
 27. A transmitter according to claim 24wherein the header OFDM symbols further contain multiplexed broadcastingchannel sub-carriers for each of the plurality of antennas.
 28. Atransmitter according to claim 24 adapted to transmit in a plurality ofdifferent modes by transmitting a different number of OFDM symbols perslot with an unchanged slot duration and with no change to the framestructure above the slot.
 29. A transmitter according to claim 28wherein modes with an increased number of OFDM symbols per slot arerealized by shortening OFDM symbol duration, and shortening FFT size,but not changing sampling frequency.
 30. A transmitter according toclaim 24 adapted to transmit to a respective set of users for each TPSframe and to signal for each TPS frame which users should demodulate theentire TPS frame.
 31. A receiver adapted to receive and process OFDMframes transmitted by the transmitter of claim
 24. 32. A method ofperforming synchronization at an OFDM receiver comprising: at each of atleast one receive antenna, sampling a received signal to produce arespective set of time domain samples; determining at least one coursesynchronization position; at each of the at least one receive antenna:a) for each of a plurality of candidate fine synchronization positionsabout one of said at least one course synchronization position: i) foreach receive antenna positioning an FFT window to the candidate finesynchronization position and converting by FFT the time domain samplesinto a respective set of frequency domain components; ii) for each saidat least one transmit antenna, extracting a respective received trainingsequence corresponding to the transmit antenna from the sets offrequency domain components; iii) for each transmit antenna, calculatinga correlation between each respective received training sequence and arespective known transmit training sequence; iv) combining thecorrelations for the at least one transmit antennas to produce anoverall correlation result for each candidate synchronization position;b) determining a fine synchronization position from the plurality ofcorrelation values; combining the fine synchronization positions fromthe at least one receive antenna in an overall fine synchronizationposition.
 33. A method according to claim 32 wherein a coursesynchronization position is determined for each receive antenna and usedfor determining the respective fine synchronization position.
 34. Amethod according to claim 32 a course synchronization position isdetermined for each receive antenna and an earliest of the positions isused determining the fine synchronization positions for all receiveantennas.
 35. A method according to claim 33 wherein the coursesynchronization position is determined in the time domain for at leastone receive antenna by looking for a correlation peak between the timedomain samples over two OFDM symbol durations.
 36. A method according toclaim 32 applied at an OFDM receiver having at least two antennas,combining the fine synchronization positions from the at least onereceive antenna in an overall fine synchronization position comprisesselecting an earliest of the fine synchronization positions.
 37. Amethod according to claim 32 wherein: sampling a received signal toproduce a set of time domain samples is done for at least three OFDMsymbol durations; determining at least one course synchronizationposition comprises performing a course synchronization in the timedomain by looking for a correlation peak between the time domain samplesreceived over two OFDM symbol durations to identify a coursesynchronization position by: a) calculating a plurality of correlationvalues, each correlation value being a correlation calculated between afirst set of time domain samples received during a first period havingone OFDM symbol duration and a second set of time domain samplesreceived during a second period immediately following the first periodand having OFDM symbol duration, for each of a plurality of startingtimes for said first period; b) identifying the course synchronizationposition to be a maximum in said plurality of correlation values.
 38. Amethod according to claim 32 wherein: combining the correlations for theat least one transmit antennas to produce an overall correlation resultfor each candidate synchronization position comprises multiplyingtogether the correlations for the at least one transmit antenna for eachcandidate synchronization position.
 39. A method according to claim 32applied to a single transmit antenna single receive antenna system. 40.A method according to claim 32 wherein the training sequence is receivedon common synchronization channel sub-carriers.
 41. A method accordingto claim 32 wherein the training sequence is received during an OFDMframe preamble.
 42. A method according to claim 32 wherein the trainingsequence is received on dedicated pilot channel sub-carriers.
 43. Amethod according to claim 42 wherein the training sequence is receivedduring an OFDM frame preamble.
 44. An OFDM receiver comprising: at leastone receive antenna; for each said at least one receive antenna, receivecircuitry adapted to sample a received signal to produce a respectiveset of time domain samples; a course synchronizer adapted to determineat least one course synchronization position; a fine synchronizercomprising at least one FFT, at least one correlator and at least onecombiner, adapted to, at each of the at least one receive antenna: a)for each of a plurality of candidate fine synchronization positionsabout one of said at least one course synchronization position: i) foreach receive antenna position an FFT window to the candidate finesynchronization position and convert by FFT the time domain samples intoa respective set of frequency domain components; ii) for each said atleast one transmit antenna, extract a respective received trainingsequence corresponding to the transmit antenna from the sets offrequency domain components; iii) for each transmit antenna, calculate acorrelation between each respective received training sequence and arespective known transmit training sequence; iv) combine thecorrelations for the at least one transmit antennas to produce anoverall correlation result for each candidate synchronization position;b) determine a fine synchronization position from the plurality ofcorrelation values; the receiver being further adapted to combine thefine synchronization positions from the at least one receive antenna inan overall fine synchronization position.
 45. A receiver according toclaim 44 having at least two receive antennas, adapted to combine thefine synchronization positions from the at least one receive antenna inan overall fine synchronization position by selecting an earliest of thefine synchronization positions.
 46. A receiver according to claim 44adapted to combine the correlations for the at least one transmitantennas to produce an overall correlation result for each candidatesynchronization position by multiplying together the correlations forthe at least one transmit antenna for each candidate synchronizationposition.
 47. A receiver according to claim 44 adapted to receive thetraining sequence on common synchronization channel sub-carriers.
 48. Areceiver according to claim 44 adapted to receive the training sequenceon dedicated pilot channel sub-carriers.
 49. A method of performing finesynchronization comprising: at each at least one receive antennareceiving OFDM symbols containing a respective received frequency domaintraining sequence for each of at least one transmit antenna; performingfine synchronization in the frequency domain by looking for maximumcorrelations between known frequency domain training sequences and thereceived frequency domain training sequences.
 50. A method oftransmitting signals enabling fine synchronization comprising: from eachof at least one transmit antenna, transmitting OFDM symbols containing arespective frequency domain training sequence.
 51. A method according toclaim 50 wherein a different frequency domain training sequence istransmitted by each transmit antenna, but the same frequency domaintraining sequence is transmitted by corresponding antenna of othertransmitters.
 52. A method of performing cell selection at an OFDMreceiver comprising: at each of at least one receive antenna, sampling areceived signal to produce a respective set of time domain samples;determining at least one course synchronization position; at each of theat least one receive antenna: a) performing a frequency domaincorrelation between at least one received common synchronizationsequence extracted from common synchronization channel sub-carriers inthe received signal and a corresponding common synchronization sequenceof a respective plurality of transmit antennas to identify a pluralityof candidate correlation peaks; b) selecting the M strongest correlationpeaks for further processing; c) at each correlation peak, reconvertingtime domain samples into frequency domain components and processingpilot channel sub-carriers, these containing transmitter specificinformation, to identify a transmitter associated with each correlationpeak; d) determining a C/I or similar value for each transmitter thusidentified; selecting the transmitter having the largest C/I determinedfor any of the at least one receive antenna.
 53. A method according toclaim 52 wherein performing a frequency domain correlation between atleast one received common synchronization sequence extracted from commonsynchronization channel sub-carriers in the received signal and acorresponding common synchronization sequence of a respective pluralityof transmit antennas to identify a plurality of candidate correlationpeaks comprises: a) for each of a plurality of candidate finesynchronization positions about one of said at least one coursesynchronization position: i) for each receive antenna positioning an FFTwindow to the candidate fine synchronization position and converting byFFT the time domain samples into a respective set of frequency domaincomponents; ii) for each of at least one common synchronizationsequence, each common synchronization sequence having been transmittedby a transmit antenna of each of at least one transmitter, extracting arespective received training sequence corresponding to the transmitantennas from the sets of frequency domain components; iii) for each ofthe at least one common synchronization sequence, calculating acorrelation between each respective received common synchronizationsequence and a respective known common synchronization sequence; iv)combining the correlations to produce an overall correlation result foreach candidate synchronization position; b) determining at least onepeak in the correlations, each said at least one peak being local maximain the correlations.
 54. A method according to claim 53 furthercomprising: reconverting time domain samples into frequency domaincomponents based on the fine synchronization position of the selectedtransmitter and performing a further fine synchronization based on adedicated pilot channel for that transmitter.
 55. A method according toclaim 54 applied to a MIMO-OFDM frame format having a header symbolformat in which subcarriers of a header symbol are divided into anon-contiguous set of subcarriers for each of a plurality of antennas,with each antenna transmitting header symbols only on the respective setof sub-carriers, and wherein the header symbols contain multiplexedpilot channel sub-carriers and common synchronization channelsub-carriers for each of the plurality of antennas, the frame beginningwith two identical header OFDM symbols during which contents of thepilot channel sub-carriers are repeated and contents of thesynchronization channel sub-carriers are repeated, the commonsynchronization channel sub-carriers carrying a complex sequence whichis different for respective antenna of one base station and being commonacross multiple base stations, and contents of the dedicated pilotchannel sub-carriers being at least locally unique to a particular basestation.
 56. A method according to claim 52 further comprising: fortransmitter switching, averaging the C/I or similar value over a timeinterval for each transmitter thus identified, and at the end of thetime interval instigating a transmitter switch to the transmitter withthe largest average C/I or similar value if different from a currentlyselected transmitted.